1. Show that triangle ABD is a right triangle.

2. Calculate DF (=4.8) (it's the height in the right triangle!)

3. Calculate AF ( = 6.4) and BF (=3.6) (use Euclid's theorem)

4. Calculate the length of CF in the right triangle AFC and calculate the length of CF in the right triangle BFC. In both cases you'll get the same result. Therefore the lines DF and CF are perpendicular on AB and meet in the same point.